t4029: use sh instead of bash

Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4029-diff-trailing-space.sh

-#!/bin/bash
+#!/bin/sh
 #
 # Copyright (c) Jim Meyering
 #